In the blending of pigments, vulcanizing aids, processing oils, plasticizers, silicones and other additives into thermoplastic resins, attaining uniform dispersions is a problem. Heated mixing machines such as dough-mixers or Banbury mixers can be used in which case the mixing is done on the melt. This requires a lot of energy. A 300 horsepower Banbury mixer can handle a 150 pound batch of polystyrene. Powdered additives or liquids can be blended into thermoplastics in the powder form using a ribbon or zig-zag blender. A masterbatch of liquid in polymer powder is often blended with more powder to get better dispersions. Extruders with mixing screws are used to blend powders while at the same time producing strands to be chopped into pellets. This does not always work well. For example, dimethylpolysiloxane gum has been blended into polystyrene by feeding gum and pellets simultaneously to an extruder. However, even after a second extrusion, blends made in an extruder are not uniform. Furthermore, pellets are not desirable for some applications. Specifically, glass reinforced thermoplastics are best made using a blend of polymer powder and the chopped glass strand, and not pellets.
There has not been discovered a simple, efficient method for blending additives, in particular, silicone gums, with thermoplastics to yield uniform mixtures in the form of powders. A solution of the thermoplastic and additive is made in a low boiling solvent, e.g., methylene chloride. This solution is then run through a conduit intimately mixed with live steam. The discharge is fed into a closed chamber and the product in the form of a damp powder is recovered at the bottom of the chamber. The powder can be further dried in an air circulating oven or by other means. It is then ready for melt fabrication into pellets or directly into a finished part. The appearance of compression molded sheets indicates that blends made in this fashion are uniform. In contrast, other mixing techniques, such as extrusion, would not produce such uniform blends.
It is believed that the system functions somewhat like a steam distillation apparatus. The polymer solution is injected into a jet of live steam and through a long tail pipe conduit. Polymer powder (crumb) comes out the bottom of the chamber, e.g., a cyclone at the end, and steam and solvent vapor go off overhead either to the atmosphere or to a condensing and solvent recovery system. Even if the additives are not completely soluble in the polymer solution, but are soluble in the solvent, the two solutions can be kept well agitated in the feed tank as they are being injected into the jet of steam.
In addition to remarkably uniform blending, the novel process has the following advantages over those described in the prior art: (i) lower energy cost; (ii) simpler equipment; and (iii) it permits intimate blending of very small amounts of additive throughout the powder particles .